In a switch mode power supply (SMPS), broadly speaking, a magnetic energy storage device such as a transformer or inductor is used to transfer power from an input side to an output side of the SMPS. A power switch switches power to the primary side of the energy storage device, during which period the current and magnetic field builds up linearly. When the switch is opened the magnetic field (and secondary side current) decreases substantially linearly as power is drawn by the load on the output side.
An SMPS may operate in either a discontinuous conduction mode (DCM) or in continuous conduction mode (CCM) or at the boundary of the two in a critical conduction mode. In this specification we are generally concerned with DCM operating modes in which, when the switching device is turned off, the current on the secondary side of the transformer steadily, but gradually, declines until a point is reached at which substantially zero output current flows. In some topologies of conventional SMPS, the inductor or transformer may begin to ring during a so-called idle phase or pause. The period of the ringing is determined by the inductance and parasitic capacitance of the circuit.
Referring now to FIG. 9, this shows an example of a SMPS circuit with primary side sensing. The power supply comprises an AC mains input coupled to a bridge rectifier 14 to provide a DC supply to the input side of the power supply. This DC supply is switched across a primary winding 16 of a transformer 18 by means of a primary switch (power switch) 20, in this example an insulated gate bipolar transistor (IGBT). A secondary winding 22 of transformer 18 provides an AC output voltage which is rectified to provide a DC output 24, and an auxiliary winding 26 provides a feedback signal voltage proportionally to the voltage on secondary winding 22. This feedback signal provides an input to a primary side sensing controller 28, powered by the input voltage, e.g., VDD. The control system provides a drive output 30 to the power switching device 20, modulating pulse width and/or pulse frequency to regulate the transfer of power through transformer 18, and hence the voltage of DC output 24. In embodiments the power switch 20 and controller 28 may be combined on a single power integrated circuit. As can be seen, the primary side controlled SMPS of FIG. 9 derives feedback information from the primary side of the transformer, using an auxiliary winding to avoid high voltage signals, the voltage being stepped down by the turns ratio of the transformer. Alternative techniques for primary side sensing (e.g., sensing a voltage of the primary winding, preferably capacitor coupled so that it can be referenced to the ground of the controller and stepped down using a potential divider, as shown by the inset example circuit of FIG. 9 with a dashed connection to the primary winding 16), and thus the auxiliary winding of FIG. 1 may be omitted.
Primary Side Sensing Controllers (PSSC) in mains-isolated Switched Mode Power Converters (SMPC) generally utilise a primary referred feedback (FB) winding WFB to sample the voltage VFB reflected to the said FB winding from the controlled secondary winding W2 of the isolation transformer during the secondary conduction interval in the converter switching cycle. The sample is then used by the control loop to vary the control quantity in the loop in order to maintain the converter output quantity equal to a reference level. A generic circuit diagram of an example asynchronous flyback converter utilizing a PSSC is shown in FIG. 1.
The voltage VFB relates to the converter output voltage Vo as follows:
                              V          FB                =                                            N              FB                                      N              ⁢                                                          ⁢              2                                ·                      (                          Vo              +                              V                F                                      )                                              Eq        .                                  ⁢        1            where N2 is the number of turns in the secondary winding, NFB is the number of turns in the FB winding, Vo is the output voltage and VF is the forward voltage drop across the rectifier. A disadvantage is that VF may become a source of error where the goal is to sample and control the output voltage Vo.
Generally, VF has two components. The first VSER is due to the series resistance of the rectifier device, the resistance of any bonding wires etc., and therefore depends on the current through the rectifier. The second component depends on the temperature and the nature of the rectification device. To minimise the error due to VSER the converter is operated in discontinuous current mode (DCM) and VFB is ideally sampled at the instant when the current through the rectifier falls to zero. Since the PSSC has no direct information on the secondary current, VFB is sampled at the onset of the idle oscillation in the transformer, at which point the rectifier current reaches zero. Usually a slope detector is used to detect this point on the VFB waveform. It is therefore desirable that in the vicinity of the sampling point the VFB is a monotonic time function in order to achieve accurate sampling. Typical waveforms for a DCM Flyback converter utilizing a diode rectifier are shown in FIG. 2.
When synchronous rectification is used with PSSC a disadvantage may arise with the sampling method described above if the synchronous rectifier is turned off rapidly. A synchronous Flyback converter and the corresponding DCM waveforms are shown in FIGS. 3 and 4, respectively.
It is evident from FIG. 4 that the rapid turn off of the synchronous rectifier (SR) triggers two undesirable events:                a) The secondary current i2(t) is switched from the MOSFET (M2) channel to its body diode, which results in a step increase in the feedback (FB) voltage.        b) A resonant transient is excited involving the output capacitance of the SR and any series inductance.        
These are disadvantageous regarding desired operation of the slope detector and potentially result in a sampling error. From a different perspective the described transient process may increase the electromagnetic interference (EMI) generated by the converter.
For use in understanding the present invention, the following disclosures are referred to:                NXP datasheet for TEA1761T Green Chip synchronous rectifier controller, at http://www.nxp.com/products/power_management/ac_to_dc_solutions/secondary_si de_controllers/TEA1761T.html.        
The field of switched mode power converters continues to provide a need for improvements in efficiency, for example where PSSC is implemented there is a need to provide synchronous rectification for improved efficiency while allowing good accuracy and/or stability of output regulation and/or reduced emission of electromagnetic interference (EMI).